A spokesman from the International Security Assistance Force today said two individuals wearing Afghan national police force uniforms carried out the attack and one of them is still at large while the other was killed in returned fire, said The Independent.

British Defense Secretary Philip Hammond told BBC today that authorities are unsure of the motive behind the attack and "don't yet know whether this was an insurgent who'd infiltrated the police or whether it was a policeman who simply had a grievance of some kind."

The killings on Saturday come amid a string of so-called "green on blue" attacks, a reference to violence directed at coalition forces by members of the Afghan security forces, said The Independent.

Such "green-on-blue" attacks now average about one a week, according to BBC defense correspondent Caroline Wyatt.

NATO earlier hinted the attackers were militants masquerading as Afghan police officers, said BBC, local authorities reported that the suspects had been members of the national force for a year.